1. Field of the Invention
This invention relates to an x-ray film processor, and more specifically to a roller film transport system for such a processor.
2. Description of the Prior Art
Unlike ordinary photographic film which is usually used and processed in strip form, x-ray film is ordinarily used and processed in separate sheets with a single frame per sheet. Processing of x-ray film typically involves movement of the film in a sinuous path through an array of rollers immersed in developing, fixing, or rinsing liquid. The rollers both guide the film through the sinuous path and press the liquid from the film somewhat in the manner of a squeegee.
In such a roller transport processor, a surface artifact generally occurs on a developing x-ray film as a line parallel to the leading edge. With rollers of 1 inch diameter, this line is 3.15 inches in from the leading edge of the film. The distance 3.15 inches is one circumference, or .pi. times the diameter of the roller, hence this line artifact is sometimes known as a "pi line". A pi line is not a defect of the film emulsion, but a deposit of particles onto the surface of the film. They can occur on either or both sides of the film, and are subject to smearing if there is slippage between rollers and film.
The deposition of particles is from the rollers onto the surface of the film. This deposition occurs in all three racks of a film processor; the developer rack, the fixer rack, and the wash rack. These particles accumulate on the rollers as a result of the chemistry in each of the racks. Particles deposit on the film because the leading edge of the film, when striking a roller, dislodges a line of particles to form a concentrated line of these particles directly in front of the film leading edge. In plain language, this may be visualized as analogous to a straight gouge, formed by an abutting straight edge, leaving displaced material in a straight mound alongside the gouge. This concentrated line or bead or loose particles remains on the roller for one roller revolution. At one revolution, when this bead of particles makes contact with the film, some particles are transferred to the film while some remain with the roller. While it is possible that such a deposition of particles from only one roller would be sufficient to make a pi line visible to the human eye, this seems unlikely. It is rather the repetition of these depositions by repeated abutments of the film leading edge on successive rollers, each such roller depositing particles on the film along the same line, that creates a visible pi line.
The quantity of particles in the chemistry increases in proportion to the number of films that are processed in it. For this reason, visible pi lines appear after a period of equipment use, and they increase in density over time as more films are processed. The current practice for avoiding pi lines, especially in industrial processors since they run at lower transport speeds than medical processors, is to clean the racks frequently and thoroughly. Some industrial processors incorporate a buffer roller in the final wash rack to wash off pi lines. It is an object of this invention to provide means to avoid or reduce pi line artifacts heretofore inherent in x-ray film processing.